Measuring devices which are implemented for progressively tracking a target point and determining the coordinate position of this point can be summarized in general, in particular in conjunction with industrial surveying, under the term laser tracker. A target point can be represented in this case by a retroreflective unit (for example, cube prism), which is targeted using an optical measuring beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is acquired using an acquisition unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the acquisition of the beam, for example, by means of runtime or phase difference measurement.
Laser trackers according to the prior art can additionally be embodied having an optical image acquisition unit having a two-dimensional, light-sensitive array, for example, a CCD or CID camera or a camera based on a CMOS array, or having a pixel array sensor and having an image processing unit. The laser tracker and the camera can be installed one on top of another in this case in particular in such a manner that the positions thereof relative to one another are not variable. The camera is, for example, rotatable together with the laser tracker about its essentially perpendicular axis, but pivotable up-and-down independently of the laser tracker and therefore arranged separately from the optic of the laser beam in particular. Furthermore, the camera—for example, as a function of the respective application—can be embodied as pivotable about only one axis. In alternative embodiments, the camera can be installed in an integrated construction together with the laser optic in a shared housing.
With the acquisition and analysis of an image—by means of image acquisition and image processing unit—of a so-called measuring aid instrument having markings, the relative location of which to one another is known, an orientation of an object (for example, a probe), which is arranged on the measuring aid instrument, in space can be concluded. Together with the specific spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or relative to the laser tracker.
Such measuring aid instruments can be embodied by so-called scanning tools, which are positioned having the contact point thereof on a point of the target object. The scanning tool has markings, for example, light spots, and a reflector, which represents a target point on the scanning tool and can be targeted using the laser beam of the tracker, wherein the positions of the markings and of the reflector relative to the contact point of the scanning tool are precisely known. The measuring aid instrument can also be, in a way known to a person skilled in the art, a handheld scanner equipped for distance measurement, for example, for contactless surface surveying, wherein the direction and position of the scanner measuring beam used for the distance measurement are precisely known relative to the light spots and reflectors which are arranged on the scanner. Such a scanner is described, for example, in EP 0 553 266.
For distance measurement, laser trackers of the prior art have at least one distance meter, wherein it can be implemented, for example, as an interferometer. Since such distance measuring units can only measure relative distance changes, so-called absolute distance meters are installed in modern laser trackers in addition to interferometers. For example, such a combination of measuring means for distance determination is known by way of the product AT 901 of Leica Geosystems AG. The interferometers used in this context for the distance measurement primarily use HeNe gas lasers—because of the long coherence length and the measuring range thus made possible—as light sources. The coherence length of the HeNe laser can be several hundred meters in this case, so that using relatively simple interferometer constructions, the ranges required in industrial measuring technology can be achieved. A combination of an absolute distance meter and an interferometer for distance determination having an HeNe laser is known, for example, from WO 2007/079600 A1.
In addition, in modern tracker systems, a deviation of the received measuring beam from a zero position is ascertained on a sensor—increasingly as a standard feature. By means of this measurable deviation, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected or tracked as a function of this deviation such that the deviation on the sensor is decreased, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking (tracking) of the target point can be performed and the distance and position of the target point can be progressively determined relative to the measuring device. The tracking can be implemented in this case by means of an alignment change of the deflection mirror, which is movable by a motor, provided for deflecting the laser beam and/or by a pivot of the targeting unit, which has the beam-guiding laser optic.
The described target tracking must be preceded by locking of the laser beam on the reflector. For this purpose, an acquisition unit having a position-sensitive sensor and having a comparatively large field of vision can additionally be arranged on the tracker. In addition, in devices of this type, additional illumination means are integrated, using which the target or the reflector is illuminated, in particular using a defined wavelength differing from the wavelength of the distance measuring means. The sensor can be implemented in this context as sensitive to a range around this specific wavelength, for example, to reduce or entirely prevent external light influences. By means of the illumination means, the target can be illuminated and, using the camera, an image of the target having illuminated reflector can be acquired. By way of the imaging of the specific (wavelength-specific) reflection on the sensor, the reflection position in the image can be resolved and therefore an angle relative to the acquisition direction of the camera and a direction to the target or reflector can be determined. An embodiment of a laser tracker having such a target search unit is known, for example, from WO 2010/148525 A1. As a function of the direction information thus derivable, the alignment of the measuring laser beam can be changed such that a distance between the laser beam and the reflector onto which the laser beam is to be locked is decreased.
One disadvantage of this locking-on operation is that, upon acquisition of the illumination radiation reflected on targets, more than one reflection from multiple different targets, which are in the range of vision of the position-sensitive sensor, is acquired and due to the ambiguity thus arising, it is not possible to reliably lock onto a desired target. It can be linked to substantial effort for a user of the measuring system in this case to align the laser beam on this desired target. For example, the coordinates of the detected reflections must be compared in this case to the possible targets and the target must be identified by the user as a function of this comparison. Such a procedure can prove to be very time-consuming depending on the number of the acquired reflections and the targets located in a measuring environment, and therefore can greatly increase the effort for initializing a measuring operation. In particular, such target locating and locking on requires qualified proficiency and experience of the user and contains in this case—especially as a function of the user qualification—substantial error sources, whereby, for example, not the desired target, but rather a further target, which is of similar design and is located close to the desired target, can be targeted and this confusion will not be noticed by the user as a result of the similarities.